Picene thin films on metal surfaces: Impact of molecular shape on interfacial coupling

The energetics at organic-metal interfaces are, to large extend, determined by the structure of the molecular adsorbate on the substrate surface. As shown by low-energy electron diffraction (LEED) the zigzag shape of picene leads to a peculiar monolayer structure on Ag(111), Ag(110), and Cu(111) with one molecule in the unit cell lying essentially flat and the other one adopting an edge-on orientation. The adsorbate-substrate coupling is weak as shown by ultraviolet photoelectron spectroscopy (UPS). This is in striking contrast to monolayers of straight pentacene, where all molecules lie flat and are strongly coupled, involving charge transfer complex formation on most of these surfaces. The peculiar packing structure of picene leads to weak interfacial coupling on clean metal surfaces. In contrast, pentacene is chemisorbed on Ag and Cu surfaces involving electron transfer from the substrate.